If you want to take I-68 you need to exit I-70 East of Washington, PA on I-79 South. You will have some steep hills on I-68 but it is a great road. We take that route all the time when we are heading East.
You can avoid the steep hills if you stay on I-70 to the PA Turnpike and then exit at Breezewood, PA and catch I-70 again. One long hill on I-70 but not as steep as on I-68.
Have done the trip both ways and prefer I-68 due to less traffic and better roads.
I don't know where you got 43 South of Pittsburgh in your directions. US 40 is in the area but it would be very slow taking that route. Long stretches of two lane road, towns, hills, etc. Scenic but I would not take that route.
Enjoy the trip to DC. Beautiful city. Lived in the area for almost 50 years.